Spinoza is setting out this proposition in order to distinguish the adequacy of the idea of the human body versus the ideas of the modifications of the human. The idea of the human body, insofar as it is related to immediacy and experience, is adequate. This concept is similar to that of a person's feelings are true insofar as they are feelings. However, the modifications of the human body which give rise to this experience are not adequate, that is, they do not have the same level of truth. In the same way as a person's feelings are true, the same person can be completely mistaken as to the nature of the causality of feelings. Here Spinoza is setting out the wisdom of honoring one's feelings in themselves, but having skepticism as to the cause of those feelings - no matter how clear that causality might seem.
Ideæ affectionum corporis humani quatenus ad humanam mentem tantum referuntur, non sunt claræ et distinctæ sed confusæ.
Translated as,
The ideas of the modifications of the human body insofar as they refer only to the human mind, are not clear and distinct but confused.
DEMONSTRATIO: Ideæ enim affectionum corporis humani tam corporum externorum quam ipsius humani corporis naturam involvunt (per propositionem 16 hujus) nec tantum corporis humani sed ejus etiam partium naturam involvere debent; affectiones namque modi sunt (per postulatum 3) quibus partes corporis humani et consequenter totum corpus afficitur. At (per propositiones 24 et 25 hujus) corporum externorum adæquata cognitio ut et partium corpus humanum componentium in Deo non est quatenus humana mente sed quatenus aliis ideis affectus consideratur. Sunt ergo hæ affectionum ideæ quatenus ad solam humanam mentem referuntur, veluti consequentiæ absque præmissis hoc est (ut per se notum) ideæ confusæ. Q.E.D.
Translated as,
For the ideas of the modifications of the human body involve the nature not only of the external bodies but also of the human body itself (by IIP16) and must involve the nature of not only of the human body but also of its parts. For the modifications are modes (by Postulate 3) by which the parts of the human body is affected and consequently the whole body. But (by IIP24 and IIP25) the adequate understanding of external bodies is not in God as the human body of component parts insofar as it considered by the human mind but insofar as an affect by other ideas. Therefore these are ideas of modifications insofar as they refer only to the human mind, just as of consequences away from premises, that is (as noted through itself) confused ideas.
SCHOLIUM: Idea quæ naturam mentis humanæ constituit, demonstratur eodem modo non esse, in se sola considerata, clara et distincta, ut etiam idea mentis humanæ et ideæ idearum affectionum corporis humani quatenus ad solam mentem referuntur, quod unusquisque facile videre potest.
Translated as,
The idea which constitutes the nature of the human mind, is demonstrated in this way that it is not clear and distinct, considered in itself, as also the idea of the human mind and the ideas of the ideas of the modifications of the human body insofar as they refer to the mind alone which each and every person is easily able to see.
